A charged particle beam apparatus such as a scanning electron microscope (hereinafter abbreviated as SEM (Scanning Electron Microscope)) or an ion microscope detects electrons emitted from a sample when irradiated with a charged particle beam such as an electron beam or an ion beam.
In particular, the scanning electron microscope for scanning an electron beam on a sample to capture an image is used for sample analysis such as observation of a fine surface shape or local composition analysis. In the SEM, an electron beam (hereinafter referred to as primary electrons) accelerated by a voltage applied to an electron source is focused by an electron lens and the sample is irradiated with the electron beam. The focused electron beam is scanned over the sample by a deflector. The emitted electrons (secondary electrons, reflected electrons) from the sample by electron beam irradiation are detected by a detector. Further, a detection signal of the emitted electrons detected by the detector is sampled at a fixed cycle. The sampling of the signals of the emitted electrons is implemented in synchronization with a scanning signal to obtain an extracted signal corresponding to each pixel of a two-dimensional image.
An intensity of the extracted signal is converted into a brightness of the image. Because an emission rate (also called yield) which is a ratio of the amount of emitted electrons from the sample to the amount of irradiated electrons varies depending on a shape of a sample surface, a difference occurs in the extracted signal to obtain a contrast reflecting the shape. In addition, the emission rate also depends on a composition and a surface potential of the sample. Hence, various contrasts other than the shape appear in the SEM image.
An identification limit dmin corresponding to a spatial resolution of an SEM image has the following relationship with a beam diameter (diameter) dp of a primary electron beam.
                    [                  Ex          .                                          ⁢          1                ]                                                                      d          min                =                  k          ·                      d            p                                              (        1        )                        where                                                      [                  Ex          .                                          ⁢          2                ]                                                            k        =                              -                          ln              (                              1                -                                  CNR                                      2                    ⁢                                                                                                                        N                            p                                                    ·                          α                                                                    ·                                              (                                                                                                            σ                              p                                                                                -                                                                                    σ                              s                                                                                                      )                                                                                                        )                                                          (        2        )            
In this example, CNR (Contrast-to-Noise Ratio) is a contrast-to-noise ratio, and empirically requires 3 to 5. N is the irradiation (dose) amount of primary electrons per pixel, α is a signal detection efficiency of emitted electrons from the sample to the detector, σp is an emission rate of an object to be measured, and σs is an emission rate of a foundation.
Therefore, if a dose amount and an emission rate difference between the object to be measured and the foundation are large as compared with a beam diameter of the given primary electron beam, k becomes small and the identification limit becomes high. Hence, in the SEM, it is important to measure the amount of emitted electrons with high accuracy.
In recent years, there have been increasing cases where samples including soft materials such as organic materials or biomaterials, semiconductor devices, and so on are to be observed by the SEM. The semiconductor device contains an electrically high resistance and insulating material and the sample is charged by the electronic beam irradiation. This leads to a problem with image defects such as an image drift during observation and disappearance of a shape contrast. In addition, in the organic material, the sample is damaged by the electron beam irradiation in addition to charging, and a change in the shape of the sample becomes problematic. As a result, a low acceleration SEM with an acceleration voltage of 1 kV or less has been put to practical use. Furthermore, in many substances, when irradiation energy of the electron beam is around 200 eV to 400 eV, the emission rate becomes a maximum value. Therefore, if an aberration of the primary electron beam can be reduced and the beam diameter can be reduced, a spatial resolution is also improved. However, when an observation area is narrowed in high magnification observation, a pixel size is reduced and the amount of electrons irradiated per pixel is increased. Therefore, even in the low acceleration SEM, there is a problem that influence of charging and damage by the electron beam irradiation becomes obvious. Therefore, observation with lower energy of the primary electron beam and a reduced electron irradiation amount per pixel is required, and highly accurate detection of emitted electrons becomes increasingly important.
In the SEM, a detector combining a scintillator and a photomultiplier tube together is used to detect the emitted electrons. When the emitted electrons collide with the scintillator, photons are generated, and the photons are guided to the photomultiplier tube by a light guide and taken out as a signal current. The signal current is converted into a signal voltage by an amplifier. When an emitted electron current decreases, an output from the photomultiplier becomes a discrete pulse, as a result of which an S/N ratio (signal-to-noise ratio) and stability are improved through the counting process. An SEM image acquiring method by the electronic counting method is disclosed in Patent Literature 1. On the other hand, when a plurality of electrons enter within a response time of the detector, those electrons may be determined as one piece in the electronic counting method. In other words, when the number of emitted electron currents increases, there are cases where the counting method may cause an undercount. For that reason, in the disclosure of Patent Literature 2, when the electrons enter within the response time of the detector, the detection is performed by an analog method.